Add context to translated strings and clarify some strings

Summary:
Many of the shorter strings are not really informative, so add context
so translators can figure out what they mean. Some of the strings were
also wrong, those have been corrected as well.

Test Plan: Still compiles.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29721